1. Field of the Invention
The invention relates generally to a system and method for engine start detection for hybrid vehicles, and more particularly to a system and method for engine start detection for the indication of the transition from electric motor drive to engine drive.
2. Background Art
Hybrid electric vehicles utilize a combination of an internal combustion engine (ICE) in connection with an electric motor that is fed electrical energy from a battery to provide the power needed to propel a vehicle (i.e., provide power to vehicle wheels) for improved fuel economy when compared to a conventional (i.e., an internal combustion engine only) vehicle. In a hybrid vehicle, fuel economy can be improved by shutting down (i.e., turning off) the engine, especially when the ICE operates inefficiently, and using the electric motor to provide all of the power needed to propel the vehicle.
However, when the user (e.g., driver of the vehicle) wants more power than the electric motor can provide or when the battery becomes depleted, a quick and smooth engine start that is substantially transparent to the driver is desirable. That is, the transition from only electric motor drive to ICE drive (or combined electric motor and ICE drive) should be seamless.
Systems and methods for determining engine starting in hybrid vehicles have been proposed. In U.S. Pat. No. 6,274,943, for example, elapse of time is measured after the current command value supplied to the generator-motor becomes less than a reference value, and it is determined whether the detected engine speed is greater or equal to a combustion-completion discriminating speed after the measured time has reached a predetermined value. Then, it is determined whether the detected engine speed is determined to be greater or equal to the combustion-completion engine speed, and if it is, it is discriminated that the engine is definitely started. However, such conventional systems and methods can have a harsh, rough, and inefficient transition from purely electric propulsion of the vehicle to other modes of powertrain operation (e.g., ICE only, or a combination of ICE and electric motor).
During the engine start, the electric motor should provide torque to turn the ICE to start speed in addition to propelling the vehicle. When the engine begins to provide torque, the motor torque should be reduced proportionally to provide a smooth transfer of torque to the wheels. When the motor torque and the engine torque are mismatched, any one or more of efficiency, smoothness and shift time may be adversely influenced using conventional approaches.
Thus, to efficiently and smoothly synchronize the transition from purely electric propulsion of the vehicle to other modes of powertrain operation (e.g., ICE only, or a combination of ICE and electric motor), there is a need for accurately determining when the engine has actually started and is producing torque.